The causes of geographic variations in the use of health care services continue to puzzle researchers. Some have proposed that physicians in geographic areas with high rates of use provide proportionally more unnecessary care than those in other areas. Available research does not support this hypothesis. Others contend that uncertainty about the effectiveness of health services leads physicians to differing conclusions about when to perform various services and is the primary cause of geographic variations. Available research also does not support this hypothesis. This article proposes a different explanation, i.e., the enthusiasm hypothesis. Currently, research data suggest that geographic differences in the use of health care services are caused by differences in the prevalence of physicians who are enthusiasts for particular services. This analysis explores the validity of the enthusiasm hypothesis using previously published data on carotid endarterectomy.